Fig. 2. Internal view through the trachea of the left side of the cardinal syrinx illustrating the effect of electrical stimulation of the ipsilateral muscles shown in the insets to the right. (a–d) Simultaneous stimulation of the m. tracheobronchialis dorsalis (dTB) and m. syringealis dorsalis (dS). In the respiratory position (a,d), the medial (ML) and lateral (LL) labia do not protrude far into the bronchial lumen (BL). Upon strong stimulation, they are adducted (move into the BL), but do not touch and close the aperture (b,c). The arrowheads outline the visible portions of the ML (white) and LL (yellow) and indicate labial extension into the bronchial lumen. When stimulation is terminated, the labia move back into the respiratory position. (e,f) Stimulation of the medial portion of the m. syringealis dorsalis (dS) alone mainly affects the ML, which is moved slightly into the lumen. White arrowheads indicate the change from the respiratory (e) to the stimulated (f) position. dS contraction may also result in a rostro-caudal stretching of the ML suggested by a change in the light reflection pattern in the video. (g,h) Stimulation of the left m. tracheobronchialis ventralis (vTB) causes abduction (movement out of the bronchial lumen) of the left LL but does not noticeably affect the position of the ML. In comparison with the opening before stimulation, the aperture of the bronchus is increased by movement of the LL in the caudo-lateral direction (downwards) in h. D, dorsal side; V, ventral side. SM, semilunar membrane.